A current communication system presupposes use of a highly selective filter to realize high reliability and high-speed information transmission with limited frequency resources. Many surface acoustic wave devices are currently being used for such applications and it is no exaggeration to say that a surface acoustic wave device is now an indispensable component which controls the efficiency of communication equipment.
The achievable performance of a surface acoustic wave device is limited by a piezoelectric substrate used, and therefore a material having large piezoelectricity and satisfactory temperature stability is required. On the other hand, since the surface acoustic wave device is used as a filter, unwanted responses which occur from various causes obstruct characteristics thereof. Therefore, a substrate material or a design technique avoiding any unwanted responses has been searched for.
FIG. 17 shows the structure of an elastic surface resonator 131 which is a typical surface acoustic wave device. The elastic surface resonator shown in FIG. 17 has a pair of reflectors 132 and an interdigital transducer (IDT) 133 arranged between this pair of reflectors and a surface acoustic wave (hereinafter also simply referred to as “SAW”) excited by the IDT is reflected at a specific frequency between the pair of reflectors and a resonant wave is thereby produced. The IDT 133 generally has a cross electrode 1332 and a dummy electrode 1333 and further a pair of common electrodes 1331 commonly connected to these electrodes.
One of unwanted responses of this element is a so-called “transverse mode” produced as an SAW propagates in a zigzag through the interdigital transducer or the reflector in an upward or downward diagonal direction (direction which is different from the direction in which the resonant wave is traveling).
Against this unwanted response, there is a technique of selecting, for example, a small WG and Wa in FIG. 17 to thereby confine the SAW within a narrow range in vertical direction in the figure (directions perpendicular to the direction in which the resonant wave is traveling) and limiting the number of transverse modes generated (first technique).
In addition, there is also a technique of assigning weights to the lengths of interdigitated portions of a cross electrode section of an IDT 143 as shown in FIG. 18 (see broken lines in FIG. 18) to make any mode other than a specific transverse mode less prone to being manifested (second technique).
However, in the case of the technique in FIG. 17 above, since WG and Wa are small, capacitance diminishes accordingly resulting in a problem that it is difficult to realize a low-loss surface acoustic wave device.
Furthermore, in the case of the technique in FIG. 18, a specific transverse mode can be controlled, yet it is difficult to suppress unwanted responses caused by other transverse modes.
Therefore, in view of the above described problems, it is an object of the present invention to efficiently suppress unwanted responses and provide a low-loss surface acoustic wave device.